He told the Huffington Post: “Relativity tells us that when we (in this case meaning all of us on Earth) observe someone/something else that is moving relative to us (in this case the International Space Station ), we will see time running slower for the moving people/things.

Space out: Scott Kelly with Mikhail Kornienko as they say goodbye to the ISS (Photo: NASA)

“It's called time 'dilation' because you can think of the moving person's time as having 'expanded' (dilated) since less time passes for them than for us."

View from above: The astronaut was in space for 340 days and returned to his family this week (Photo: NASA)

He said a complex equation works out that one year in orbit is equivalent to 0.9999999997 years on Earth so the rest of us experienced a year, Kelly experienced just a tiny bit less than one full Earth year.

Specifically, his year was shorter than ours by 0.0000000003 years, or roughly 0.01 seconds.